1. Field of the Invention
The present invention relates to a method for separating manganese from magnesium and removing manganese from sulfuric acid acidified wastewater which contains aluminum, magnesium, and manganese simultaneously.
2. Description of the Related Art
Wastewater generating at factories sometimes contains heavy metals originating from processes or from raw materials, and cannot be discharged to the outside of a system, such as to a river and to a sea area, without any treatment. Therefore, used is, for example, a method shown in Japanese Unexamined Patent Publication No. 2005-125316, in which the above-mentioned wastewater is sent to a wastewater treatment process, and a neutralizing agent is added thereto to adjust pH, and contained metal ions are made into a precipitate form of hydroxide or the like and separated from the wastewater, and then the wastewater after separated from heavy metals is discharged to the outside of a system.
However, when heavy metals are separated from wastewater as a precipitate, the heavy metals is not easily fixed in the precipitate stably, and the treatment sometimes takes a great deal of time and effort and cost. For example, in the case of acidic wastewater containing manganese, in order to stably fix manganese as a hydroxide precipitate, a pH of the wastewater has to be adjusted in a range of 8 to 10. To this end, it is necessary that, by adding a large amount of alkaline slurry of calcium hydroxide or the like, neutralization is performed to make the wastewater alkaline from strongly acidic.
Furthermore, for example, in the case of wastewater which generates after recovering valuable metals by acid-leaching of ore, besides manganese, magnesium is also often contained. Magnesium does not affect environment, and therefore, unlike manganese, does not need to be removed from wastewater, but, at the time of a precipitation treatment of manganese, a precipitate containing magnesium is formed simultaneously, and therefore a neutralizing agent with an amount required for generating the precipitate of magnesium is consumed, and accordingly a neutralizing agent with an excessive amount more than a chemical equivalent required for generating manganese hydroxide is required. Thus, an increase in an amount of a neutralizing agent consumed causes not only cost rise but also an increase in an amount of a generated precipitate, thereby increasing the time and effort of the treatment, and therefore is not preferable. Further, when an amount of the precipitate increases, a manganese grade in the precipitate relatively decreases, and there arises a harmful effect which is such that reuse of manganese as resources becomes disadvantageous in cost.
Therefore, for example, Japanese Unexamined Patent Publication No. 3225836 discloses a method for preferentially removing manganese from an acidic solution containing magnesium and manganese.
The method disclosed in Japanese Unexamined Patent Publication No. 3225836 is a method wherein a neutralizing agent is added to an acidic solution, which is obtained by acid-leaching a nickel oxide ore and contains 1 to 3 g/L of manganese and 2 to 15 g/L of magnesium, to adjust a pH of the solution in a range of 8.2 to 8.8, while air, oxygen, ozone, or peroxide is added thereto so that an oxidation-reduction potential (ORP) is adjusted to be maintained at a range of 50 to 300 mV in an electric potential measured with a silver-silver chloride electrode, whereby manganese is preferentially precipitated and removed.
Since use of this method enables manganese to be removed prior to magnesium, consumption of a neutralizing agent for precipitate generation of magnesium can be controlled, and an amount of the neutralizing agent used can be saved.
However, a solution shown in Japanese Unexamined Patent Publication No. 3225836, which is obtained after an sulfuric acid acidified solution obtained by leaching a nickel oxidation ore with sulfuric acid is sulfurized to separate nickel as a sulfide, often contains aluminum besides magnesium and manganese, and when a wastewater treatment is performed for such an acidic aqueous solution containing aluminum, even by using the method disclosed in Japanese Unexamined Patent Publication No. 3225836, a considerable amount of magnesium also precipitates together with manganese, and a neutralizing agent is excessively required.
The excessive use of a neutralizing agent is described with an example thereof. First, a sulfuric acid acidified aqueous solution having a manganese concentration of 3.4 g/L, a magnesium concentration of 8.4 g/L, and an aluminum concentration of 0.5 to 2.0 g/L is placed into a beaker having a capacity of 0.2 liters, and a solution temperature is maintained at 50 degrees C. by using a water bath. Next, a calcium hydroxide slurry having a concentration of 20% by mass is added to the aqueous solution to adjust pH to 8.0, and when the pH becomes stable, while an oxidation-reduction potential (ORP) of the solution is measured by using a silver-silver chloride electrode as a reference electrode, pure oxygen gas is blown from a cylinder into the solution in the beaker at a flow rate of 1 liter per minute, followed by maintaining for 30 minutes. The oxidation-reduction potential at that time was around 200 mV. Then, the solution in the beaker was solid-liquid separated, and a recovered precipitate was analyzed to calculate a removal amount of aluminum and magnesium.
As shown in FIG. 1, with a molar ratio of magnesium to aluminum in the precipitate (Mg/AI) being maintained at around 2, magnesium also precipitates, whereby it is understood that a neutralizing agent is excessively required.
Thus, due to wastewater containing aluminum, the treatment method described in Japanese Unexamined Patent Publication No. 3225836, in which the above-mentioned oxidation and neutralization are performed, cannot control coprecipitation of magnesium with manganese, and therefore is uneconomical. Accordingly, there has been sought a method wherein, while precipitation of magnesium being controlled, manganese is precipitated and separated from a sulfuric acid acidified solution containing aluminum, magnesium, and manganese.
Therefore, the present invention provides a wastewater treatment method for efficiently removing manganese selectively from sulfuric acid acidified wastewater containing aluminum, magnesium, and manganese.